Rupert (W.Yorkshire) wrote:
The idea of green manures is to lock up nutrients, from both the air and
soil, in the plant. The plant is then dug in and after decaying it releases
its goodies to the soil.
The decaying process takes time and consequently if veg/plants are sown too
soon then they will in effect be growing in depleted soil.
This explanation might fit the poor results you commented on.

I also suspect that the decay process /requires/ nitrogen, thus
removing a small amount of existing nutrition until it is complete. I
suspect that adding urine might help, and may have accompanied animal
husbandry when this was used commercially.